The present invention relates to a method and an apparatus for surveying the surface of an object.
In the prior art, methods and apparatus for contactless determination of surface profiles of objects are known. One such method consists of projecting a periodic pattern onto the surface to be determined and evaluating a grating image deformed by the shape of the surface to compute the height of the points of the object surface with respect to a reference plane. For generating such a pattern of stripes, various methods are known. As shown by C. L. Koliopoulos in "Interferometric Optical Phase Measurement Techniques" (Dissertation, Univ. of Arizona, 1981), the stripe patterns are generated by means of a Mireau-Interferometer or a Twyman-Green-Interferometer. There, the breadth of the stripes or the period of the pattern is then essentially given by the wavelength of the light used for illumination.
As disclosed in published European Pat. application No. 182,469, the patterns of stripes may also be generated by a Shearing-Interferometer or by projection of a sine grating. In this case, the period may be determined by adjusting the shearing angle or by choosing a suitable sine grating. The in-depth resolution of such a surface measurement is mainly given by the period of the pattern of stripes.
A characteristic problem of the known methods arises if the absolute height of the object points must be found with respect to a reference plane that is fixed in space. While the height of each single object point within one stripe of the projected pattern is conveniently determined with respect to the lower edge of this stripe, i.e., there is an ambiguity in the height measurement corresponding to the period of the pattern. In order to obtain the absolute height of each object point a great number of stripes generally must be counted, which separate the respective object point from the reference plane.